Why were guns invented and did the person not mind all the deaths that would come with the invention ?

I sometimes wonder what people who helped develop the bombs that dropped on Japan feel? Im not sure if he would feel bad or not. Perhaps As he aged or in his final years? Perhaps he thought maybe having those weapons helped expand his nation? Maybe the wars were shorter? I feel like if he saw the raid firing guns of today and the culture of this era it might effect him more.